In the case of the mutually bonding of different products that have been molded using the thermoplastic resin, the known methods utilize ultrasonic waves, high frequency electromagnetic induction or the bonding agents.
However, in these bonding methods, a great amount of cost is involved with the installation in the case of utilizing the ultrasonic waves and high frequency electro-magnetic induction. Moreover there is a problem of inducing a defective factor to be present in the electronic parts in the situation involving incorporation of these electronic parts inside the case that is a molded product. In addition, the coupling by use of the bonding method not only uses the time until it is hardening but also may cause stain due to the leakage of bonding agent from the mated faces. This can result in a defect that affects the outside appearance unfavorably.
Therefore, a variety of such methods are presented such as disposing a heating element (hereinafter to be called "Resistance Heating Element") based on the electric resistance between two molded products being bonded, and fusing the thermoplastic resin by this exothermic phenomenon to mutually bond both the molded products.
This method is practically an extremely effective bonding method because a simple installation is available without exerting any influence on the internal parts and the outside appearance since this method is the bonding by use of instantaneous heat.
As the method for impressing the voltage to the resistance heating element in the bonding method in general includes inducing the resistance heating element from the molded product to form the voltage impressed portion and butting the electrode against this voltage impressed portion.
These kinds of voltage impression methods are disclosed by this patent applicant in Japanese Patent Application No. H8-209647, Japanese Patent Application No. H8-280435, Japanese Patent Application No. H9-136627 and others. These impression methods are the methods of previously providing on the molded product the electrode inserting hole reaching the resistance heating element caught between the bonding faces of the molded product, inserting the electrode into this inserting hole to have it contact the resistance heating element and impressing the voltage to bond it. According to this method, the resistance heating element cannot be seen from the outside, which can act to enhance the sealing property.
In this way, the bonding method using the resistance heating element is an excellent method, but has the problems as follows:
For making the bonding electrode contact with the resistance heating element for securely impressing the voltage, an adequate push force needs to be applied on the electrode. For example, in case of the bonding machine fabricated by the inventor of this invention, the push pressure of 3 N/mm.sup.2 is applied on the electrode.
Therefore, when the resistance heating element gets heated up to fuse the surrounding resin, the resistance heating element at the portion to which the push pressure is applied to the bonding electrode gets buried sometimes into the fused resin or the receptacle portion abnormally deformed. Furthermore, because the pressure impressed portion of the resistance heating element gets softened and deformed by the heat generation, the contact defect with the electrode is caused failing to obtain the satisfactory bonding.
For this reason, it is important in the heat bonding method using an electrode that the heat generation should be suppressed in the voltage impressed portion even if the resistance heating element should generate the heat so that the position and shape of voltage impressed portion may not get deformed.
Therefore, in the past, such a steps are taken, in the mounted portion of voltage impressed portion of resistance heating element with which the electrode comes in contact, as increasing more the wall thickness of resin more than that of the other mounted portions, and as adjusting the delicate balance between the fusion time and the push pressure of electrode in addition to the molded product structural measure of the addition, etc. of a holding rib so that the mounted portion may not get deformed.
However, as to increasing the wall thickness of the resin and the structure of a holding rib, there may arise the situation such that the steps cannot be taken due to structural problems like the parts and internal shape, etc. being incorporated into the molded product. Also, if the wall thickness of molded product should be thin, the product face which is the rear face of the voltage impressed portion of resistance heating element tends to slightly heap up when other molded product thermally bonded to the rear face by such reasons as the voltage impressed portion of resistance heating element gets heated up higher than the other portions attributable to the push pressure of a bonding electrode and the contact resistance. Especially in case of the product face that has been coated with luster paint, this heap-up phenomenon appears conspicuously in some cases and the product cannot be satisfactorily accepted in view of its design.